SY said:
Yes, I believe that is pretty much correct. There are probably some other time constants involved, but they are likely much less significant. I'm not much good at estimating heat capacities to arrive at the corresponding thermal time constants, however. But they can be measured. I would also tend to guess very roughly that the thermal "mass" of an element is somewhat proportional to its physical mass.
If you look at the SOA vs pulse time for a MOSFET, for example, you can see that it implies that the fastest time constants are measured in milliseconds.
By the same token, its pretty obvious that the time constant of any decent amplifier heat sink is in the minutes.
Cheers,
Bob
Bob Cordell said:
A quote from somewhere:
"[…] Just when I thought my math skillz could not get dumbified any comes a “tip guide” on a recent receipt of mine. I received the bill, and as usual, tried calculating what 18% was, but while calculating this mind exercise I noticed on the bottom of the bill was a “tip guide” that already did the calculations at 10, 15, and 20% of the bills cost. As much as I love this little added service, I worry that I’ll become dumb with math, much like what spell-check has done to my mad spelling skillz as I mentioned in my post about dumbifying tech. […]"
For the use of others, you are attempting to create kinda "tip guide". Perhaps good for them, others prefer to do the math.
I believe I can explain what you're missing.
You'll notice no reference to absolute temperature is made, we are dealing solely with relative temperatures.
For convenience I'll be using a system with the following constants Tr=0.5 Tcvbe=-0.0022 Gm=10 Vce=45.4545, this gives us a beta of 0.5. When the ambient temperature goes up one degree the Pd goes up by 1 watt and the temperature rises by an additional 0.5 degree, this is important, the temperature does not rise 1.5 degrees, it rises 0.5 degrees relative to ambient which happens to have risen by 1 degree already. Now that it has risen an additional 0.5 degree we can plug the numbers and see what further increase in temperature we get, in this case another 0.25 degree.
Already we can see the arithmetical series emerging:
1+1/2+1/4+1/8...1/n. This of course equals exactly 2, which is also what was predicted by the standard feedback equations presented by Bob.
P.S. It should of been obvious, but I only just noticed that the relevant series will always be 1+beta^1+beta^2+beta^3...+beta^n
Bob Cordell said:
Hi Bob,
This all makes a lot of sense but let me just mention a few things that might not be obvious. We see a lot of discussion about the selection of the emitter resistor based on distortion and optimal bias current but I think that you are pointing out that this resistor serves another important function. Not only does it provide local feedback for the signal, it also provides local feedback for the thermal runaway problem. We have two factors working together to avoid thermal runaway, the counter voltage produced across the emitter resistor which is fast, and the much slower thermal sensor based feedback. The bias current will tend to start to run away because of the delay in the slow outer loop, then as the heat sink heats the outer feedback loop corrects, but the die has to heat up significantly before the heat sink catches up, so there is a significant lag in the system.
Your point, as I understand it, is that if we do not have enough local thermal feedback supplied by the emitter resistor, the bias current can runaway to full latch up before the outer loop has time to respond. Your equation allows a minimum emitter resistor value to be calculated, depending on circuit parameters, and thermal factors, so the local feedback is enough to avoid destructive runaway.
The point that you're making, as I understand it, is that the bias overshoot magnitude, while the slow outer loop catches up, is a function of Beta. And it can be so bad the the bias overshoots to a destructive Ic before the slow outer loop has had a chance to correct.
I think part of the problem some are having here is that they do not even consider the emitter resistor to be part of the thermal feedback system.
One implementation detail to consider, at least for T03 output devices, is that many mount the thermal sensor thermally coupled to the metal case rather than the heat sink. This avoids some of the thermal resistance issues that you've mentioned.
I suspect that this amp, (+/- 42V rails) probably had issues with runaway under certain conditions:
http://www.diyaudio.com/forums/attachment.php?s=&postid=484803&stamp=1096758143
Pete B.
>http://www.diyaudio.com/forums/attachment.php?s=&postid=484803&stamp=1096758143
That's because it's a 'common emitter' output stage.
Thermal stuff is amplified to the extent of the voltage gain of the stage.
That's because it's a 'common emitter' output stage.
Thermal stuff is amplified to the extent of the voltage gain of the stage.
.
john curl said:
The discussion concerns thermal bias stability and ways of assessing it. This is of concern both from the point of view of unwanted bias movements in the output stage with program and with respect to the possibility of thermal runaway.
I put forth a simple formula that gives an estimate of one aspect of thermal bias stability, and Charles is questioning its correctness and applicability. Is this discussion boring you?
Actually, I think that if you run my formula on the output stage of the JC-1 you get a Beta value of about 0.64 on the original design at high bias with 0.1 ohm RE's. This is a bit into the danger zone. Didn't you mention quite awhile back that you backed off a bit due to bias stability concerns? I don't remember if you said you went to higher RE (like maybe 0.15 ohm) or what, but I do seem to remember something.
Cheers,
Bob
I have found it difficult to get thermal tracking with 10 ohm base resistors. I had to make one unit work, by personally matching the bipolar output devices, myself. Before, even with .15 ohms, this amp would go into current hogging and ultimately thermal runaway. I also worked with the factory on better thermal management of the heatsink in order to make it more even. It's fixed now.
Personally, I have found that the overshoot is proportional mostly to the distance of the temp sensors from the thermal action. Far away is BAD!
Personally, I have found that the overshoot is proportional mostly to the distance of the temp sensors from the thermal action. Far away is BAD!
john curl said:
Hi John,
VERY good point about the distance of the sensors from the thermal action. We'd probably love it if it were, but the big heatsink is definitely not isothermal.
I'm curious about the 10-ohm base resistors. Was it the presence of these resistances that led to the need to match the transistors for Beta (I'm presuming you were matching them for Beta as opposed to Vbe)?
I guess a transistor biased at 150 mA with a Beta of 75 would have a base current of 2 mA, which would drop 20 mV across the 10 ohm base resistor. So if you had transistors that ranged in Beta from 50 to 100, your spread of drops would correspond to enough delta mV across 0.1 or 0.15 ohms to cause a bit more imbalance in bias current than you would like. Just thinking out loud here.
Also, what led to your selection of 10-ohm base-stopper resistors? I honestly don't have a lot of experience with RET output stages, but the value of 10 ohms strikes me as a bit high for bipolars. Can you shed some light on this?
Thanks,
Bob
Hi John,
When we were using Roedersteins, we used 1 ohm. For our more expensive amps we have switched to the PRP audio resistors. The smallest value they make is 2 ohms, so that is what we use for our expensive amps.
Of course the base stopper also affects stability into capacitive loads. So if you change the base stopper value, you may need to re-do your compensation values slightly. I never got much help from the base stoppers in this regard. I tried going as high as 100 ohms for the output transistors, even though this completely screwed up the output impedance, but didn't get much help. So I ended going a different route for stability into capacitive loads and a smaller value of base stopper to keep the output impedance down.
I've never had the guts to try 0 ohms for the base stoppers, but who knows, maybe that would be best of all?
PS -- Never thought about the value of the base stoppers from a thermal runaway point of view, probably because we've never had a problem in that regard.
Cheers,
Charlie
When we were using Roedersteins, we used 1 ohm. For our more expensive amps we have switched to the PRP audio resistors. The smallest value they make is 2 ohms, so that is what we use for our expensive amps.
Of course the base stopper also affects stability into capacitive loads. So if you change the base stopper value, you may need to re-do your compensation values slightly. I never got much help from the base stoppers in this regard. I tried going as high as 100 ohms for the output transistors, even though this completely screwed up the output impedance, but didn't get much help. So I ended going a different route for stability into capacitive loads and a smaller value of base stopper to keep the output impedance down.
I've never had the guts to try 0 ohms for the base stoppers, but who knows, maybe that would be best of all?
PS -- Never thought about the value of the base stoppers from a thermal runaway point of view, probably because we've never had a problem in that regard.
Cheers,
Charlie
Charles, when I made my first RET amplifier back in 1980 or so, I first left out the 10 ohm resistors. Then, John Iverson gave me the suggestion to use them, because I was getting random problems. It worked and I was able to get over 500V/us per side, so I never thought much of the value as being too much and it DOES add a positive R which cancels the negative R is being generated by the cap load and the transistor.
However, in analysis of beta balance, I found that 10 ohms to be problematic. I would be happy with 4-5 ohms. That would help me, and probably still keep me safe. After all, like you, I don't use an output coil, but I do use a significant amount of negative feedback, so I have to be more careful.
However, in analysis of beta balance, I found that 10 ohms to be problematic. I would be happy with 4-5 ohms. That would help me, and probably still keep me safe. After all, like you, I don't use an output coil, but I do use a significant amount of negative feedback, so I have to be more careful.
john curl said:
Oh, I WISH it were that simple. By far the best book on this is a long out-of-print and very hard to find book by Dennis Feucht. He explains it in more detail than anything I found. But everyone basically said the same thing -- add a positive R value to cancel the effective negative R created by the capacitive load.
So I tried crazy values of R, up to a hundred ohms on the outputs and thousands of ohms on the drivers. Barely helped at all, and it totally screwed up everything else about the circuit. I began to see why so many people use an isolation coil...